Several disorders arising from hyposecretion of one or more substances such as hormones are known. Among these are diabetes, Parkinson's disease, Alzheimer's disease, hypo- and hyper-tension, hypothyroidism, and various liver disorders. The hormone insulin, for example, is produced by β-cells in the islets of Langerhans of the pancreas. In normal individuals, insulin release is regulated so as to maintain blood glucose levels in the range of about 70 to 110 milligrams per deciliter. In diabetics, insulin is either not produced at all (Type 1 diabetes), or the body cells do not properly respond to the insulin that is produced (Type 2 diabetes). The result is elevated glucose levels in the blood.
Disorders arising from hyposecretion of a hormone are usually treated by administration of the missing hormone. However, despite advances in understanding and treating many of these diseases, it is often not possible to precisely regulate metabolism with exogenous hormones. A diabetic, for example, is required to make several daily measurements of blood insulin and glucose levels and then inject an appropriate amount of insulin to bring the insulin and glucose levels to within the acceptable range.
Organ transplantation is not a viable treatment today for most of these disorders for several reasons including rejection of a transplanted organ by the immune system. Isolated cells may be implanted in the body after being treated to prevent rejection. e.g. by immunosuppression, radiation or encapsulation. Methods of immunoprotection of biological materials by encapsulation are disclosed in U.S. Pat. Nos. 4,352,883, 5,427,935, 5,879,709, 5,902,745, and 5,912,005. The encapsulating material is selected so as to be biocompatible and to allow diffusion of small molecules between the cells of the environment while shielding the cells from immunoglobulins and cells of the immune system. Encapsulated β-cells, for example, can be injected into a vein (in which case they will eventually become lodged in the liver) or embedded under the skin, in the abdominal cavity, or in other locations. Fibrotic overgrowth around the implanted cells, however, gradually impairs substance exchange between the cells and their environment. Hypoxygenation of the cells ultimately leads to cell death.
U.S. Pat. No. 5,855,613 discloses embedding cells in a thin sheet of alginate gel that is then implanted in a body.
U.S. Pat. No. 5,834,005 discloses immunoisolating cells by placing them in a chamber that is implanted inside the body. In the chamber, the cells are shielded from the immune system by means of a membrane permeable to small molecules such as glucose, oxygen, and the hormone secreted by the cells, but impermeable to cells and antibodies. These implanted chambers, however, do not have a sufficiently high surface area to volume ratio for adequate exchange between the cells and the blood. Thus, also in this case, hypoxygenation ultimately leads to cell death.